* [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'
@ 2021-11-03 21:35 Christophe JAILLET
2021-11-03 21:41 ` Matthew Wilcox
2022-08-12 4:58 ` Christophe JAILLET
0 siblings, 2 replies; 5+ messages in thread
From: Christophe JAILLET @ 2021-11-03 21:35 UTC (permalink / raw)
To: akpm; +Cc: linux-mm, linux-kernel, kernel-janitors, Christophe JAILLET
Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
save a few cycles when it is known that the rcu lock is already
taken/released.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
mm/memremap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/memremap.c b/mm/memremap.c
index 84de22c14567..012e8d23d365 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
/* fall back to slow path lookup */
rcu_read_lock();
pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
- if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
+ if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
pgmap = NULL;
rcu_read_unlock();
--
2.30.2
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'
2021-11-03 21:35 [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()' Christophe JAILLET
@ 2021-11-03 21:41 ` Matthew Wilcox
2021-11-03 21:54 ` Christophe JAILLET
2022-08-12 4:58 ` Christophe JAILLET
1 sibling, 1 reply; 5+ messages in thread
From: Matthew Wilcox @ 2021-11-03 21:41 UTC (permalink / raw)
To: Christophe JAILLET; +Cc: akpm, linux-mm, linux-kernel, kernel-janitors
On Wed, Nov 03, 2021 at 10:35:34PM +0100, Christophe JAILLET wrote:
> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> save a few cycles when it is known that the rcu lock is already
> taken/released.
If this is really important, we can add an __xa_load() which doesn't
take the RCU read lock.
I honestly think that the xarray is the wrong data structure here,
and we'd be better off with a simple array of (start, pointer)
tuples.
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
> mm/memremap.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 84de22c14567..012e8d23d365 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> /* fall back to slow path lookup */
> rcu_read_lock();
> pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
> - if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
> + if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
> pgmap = NULL;
> rcu_read_unlock();
>
> --
> 2.30.2
>
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'
2021-11-03 21:41 ` Matthew Wilcox
@ 2021-11-03 21:54 ` Christophe JAILLET
2021-11-03 22:01 ` Matthew Wilcox
0 siblings, 1 reply; 5+ messages in thread
From: Christophe JAILLET @ 2021-11-03 21:54 UTC (permalink / raw)
To: Matthew Wilcox; +Cc: akpm, linux-mm, linux-kernel, kernel-janitors
Le 03/11/2021 à 22:41, Matthew Wilcox a écrit :
> On Wed, Nov 03, 2021 at 10:35:34PM +0100, Christophe JAILLET wrote:
>> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
>> save a few cycles when it is known that the rcu lock is already
>> taken/released.
>
> If this is really important, we can add an __xa_load() which doesn't
> take the RCU read lock.
There are a few:
rcu_read_lock();
mem = xa_load(...);
rcu_read_unlock();
patterns here and there.
I don't have any numbers of if saving some rcu_read_lock/rcu_read_unlock
would be useful in these cases.
The only numbers I have are in [1].
[1]:
https://lore.kernel.org/linux-kernel/cover.1634822969.git.asml.silence@gmail.com/
CJ
>
> I honestly think that the xarray is the wrong data structure here,
> and we'd be better off with a simple array of (start, pointer)
> tuples.
>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
>> ---
>> mm/memremap.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 84de22c14567..012e8d23d365 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>> /* fall back to slow path lookup */
>> rcu_read_lock();
>> pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
>> - if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
>> + if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
>> pgmap = NULL;
>> rcu_read_unlock();
>>
>> --
>> 2.30.2
>>
>>
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'
2021-11-03 21:54 ` Christophe JAILLET
@ 2021-11-03 22:01 ` Matthew Wilcox
0 siblings, 0 replies; 5+ messages in thread
From: Matthew Wilcox @ 2021-11-03 22:01 UTC (permalink / raw)
To: Christophe JAILLET; +Cc: akpm, linux-mm, linux-kernel, kernel-janitors
On Wed, Nov 03, 2021 at 10:54:09PM +0100, Christophe JAILLET wrote:
> Le 03/11/2021 à 22:41, Matthew Wilcox a écrit :
> > On Wed, Nov 03, 2021 at 10:35:34PM +0100, Christophe JAILLET wrote:
> > > Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> > > save a few cycles when it is known that the rcu lock is already
> > > taken/released.
> >
> > If this is really important, we can add an __xa_load() which doesn't
> > take the RCU read lock.
>
> There are a few:
> rcu_read_lock();
> mem = xa_load(...);
> rcu_read_unlock();
> patterns here and there.
If that's all they are, then the rcu_read_lock() and unlock can be
deleted.
if they're actually
rcu_read_lock()
mem = xa_load(...);
try_get_ref(mem);
rcu_read_unlock();
then of course they can't be.
> I don't have any numbers of if saving some rcu_read_lock/rcu_read_unlock
> would be useful in these cases.
>
> The only numbers I have are in [1].
>
> [1]: https://lore.kernel.org/linux-kernel/cover.1634822969.git.asml.silence@gmail.com/
It may not be worth hyperoptimising the slow path like this patch ...
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'
2021-11-03 21:35 [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()' Christophe JAILLET
2021-11-03 21:41 ` Matthew Wilcox
@ 2022-08-12 4:58 ` Christophe JAILLET
1 sibling, 0 replies; 5+ messages in thread
From: Christophe JAILLET @ 2022-08-12 4:58 UTC (permalink / raw)
To: akpm; +Cc: linux-mm, linux-kernel, kernel-janitors
Le 03/11/2021 à 22:35, Christophe JAILLET a écrit :
> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> save a few cycles when it is known that the rcu lock is already
> taken/released.
>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
> mm/memremap.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 84de22c14567..012e8d23d365 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> /* fall back to slow path lookup */
> rcu_read_lock();
> pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
> - if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
> + if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
> pgmap = NULL;
> rcu_read_unlock();
>
Hi,
gentle reminder.
Is this patch useful?
When I first posted it, percpu_ref_tryget_live_rcu() was really new.
Now it is part of linux since 5.16.
CJ
^ permalink raw reply [flat|nested] 5+ messages in thread
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2021-11-03 21:35 [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()' Christophe JAILLET
2021-11-03 21:41 ` Matthew Wilcox
2021-11-03 21:54 ` Christophe JAILLET
2021-11-03 22:01 ` Matthew Wilcox
2022-08-12 4:58 ` Christophe JAILLET
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